Sheetpiling has been used in the construction industry for over 200 years (for example) to support excavations, create cut-offs and stabilize ground slopes. The sheetpiling can be used as either a free-standing structure or used in conjunction with tie-backs, props or ground anchors. The earth pressure and groundwater forces on the sheetpiles are dispersed along and across the sheetpiles making flexural strength of the sheetpile the main factor in design of the sheetpile.
Conventional sheetpiling consists of hot rolled steel sections (+5 mm thick) manufactured to `unit` profiles which are linked through interlocking joints to create composite structures. Since 1933, a variety of light sheetpiles have been developed using thin (t=&lt;5 mm), steel plate cold formed or rolled into lighter versions of conventional sheetpile profiles. Some use has also been made of pleated or corrugated profiles as light sheetpiles.
Sheetpiling can be divided into two types representing
(a) the conventional sheetpiles (t&gt;5 mm) made up of one or two basic bends to create a U or Z profile in a narrow (ws&lt;600 mm) sheetpile that are linked with interlocking joints to form a repetitive and/or deeper section profile; and PA1 (b) the light (t=&lt;5 mm) sheetpiles made up of a pleated, corrugated or trapezoidal profiles that repeat across a wider sheet (w=400 to 800 mm), where (t) is the thickness of the material and (w) is the effective width of the sheetpile). PA1 Profile sizing parameters (w, d, t, f, i, f/d, n) which relate to and define the characteristics of the stiffening panel means, driving rib means and joint strip means of the sheetpile; and PA1 overall sizing parameters (ws/tm, N,) which relate to the overall configuration of the sheetpile
Current sheetpile types are a compromise between structural capacity, lateral stability, joint design, driving capacity, manufacture and construction aspects. The types of sheetpile can be categorized by section profile parameters (d, f, i, w, n and t) and overall sheet parameters (Ws, N) as defined further below.
Conventional sheetpiles are usually made up in a single `U` or `Z` profile linked through the joint between sheetpiles to form a symmetrical section profile. The eccentric loads created during driving and loading of these unsymmetrical sheetpile units can be accommodated by the heavier construction of conventional (t&gt;5 mm) sheetpiles. However, light (t=&lt;5 mm) sheetpiles have to be formed to a symmetrical section profile to avoid eccentric loads during driving and loading of the sheetpiling. Thus the full profile has to be formed within each sheetpile, including the jointing system. On light sheetpiles (t=&lt;5 mm), the lateral load distribution across the sheetpile has been a factor in limiting the sizing, and effective width of the profile. These two requirements have been key factors in restricting the sizing section profiles of light sheetpiles.
The structural form of the section profile adopted for sheetpiling can be related to a flange width (f) to section depth (d) by the (f/d) ratio and the web inclination (i). These parameters fit within specific ranges which determine the structural performance of the sheetpiling. Conventional sheetpiles have adopted a limited range of flange widths (f) which results in a progressive decrease in the f/d ratio as the section depth (d) increases, viz:
______________________________________ DEPTH (f) RATIO (mm) (d) ______________________________________ &lt;120 1.6 &lt; f/d &lt; 4.0 120 &lt; d &lt; 250 0.8 &lt; f/d &lt; 2.4 250 &lt; d &lt; 450 0.5 &lt; f/d &lt; 1.5 450 &lt; d no examples ______________________________________
Light (t=&lt;5 mm) sheetpiles have adopted a wide range of (f/d) ratios (0.3&lt;f/d&lt;3.5) due to the shallow (d&lt;100 mm), profiles used in this type of sheetpile. The web inclination (i) verses (f/d) ratio reflects the limits existing on the (f/d) ratio and section depth (d).
Lateral stiffness and strength of the sheetpile control the sheetpile's effective width (ws) and thickness (t). A survey of typical sheetpiling systems indicates that conventional sheetpiles lie within sheetpile width to thickness (ws/t) ratios of 20 to 140. The (ws/t) ratio for light (t=&lt;5 mm) sheetpiles ranges from 40 to 190. Structural Codes impose upper limits of 60 to 100 on (ws/t) ratios, although (ws/t) ratios up to 180 can be allowed in the web section of the steel beams. At the higher ratio (ws/t&gt;100), steel structures encounter both lateral strength and stability problems.
Wider sheetpiles (ws=800 mm, ws/t&gt;150) suffer excessive rotational deformations (.differential./w&gt;0.1), with lateral movements (.differential.) at the edges of sheetpiles of 80 to 100 mm occuring, even in sheetpiles supporting shallow excavations (4 m ) into favourable ground conditions. Thus overall use has imposed a limit on (ws/t) ratio of 150 to prevent a lack of lateral stiffness and stability problems. Furthermore, flexural tests on corrugated sheets have shown that load transfer across a light sheetpile becomes negligible once the (ws/t) ratio exceeds 150. Thus the outer corrugations do not contribute to the longitudinal flexural strength, due to `curling-up` of the edge of the sheet. These tests supported the limit of 100 to 150 on the (ws/t) ratio suggested in the various Structural Codes.
The overall integrity of a sheetpile system also depends on the joint system, driving capabilities and impermeability of the insitu sheetpiling. These three factors are not usually designed, but have developed from manufacturing requirements and field experience.
The joint systems used along the edge of sheetpiles can be divided into simple `overlap` joints, the `hooked` joint and the `interlocked` joint. The joints are formed as an integral part of the section profile in both conventional (t&gt;5 mm) and light (t=&lt;5 mm) sheetpiles. Conventional (t&gt;5 mm) sheetpile use `interlocked` joints based on a `claw-paw` design moulded into the edge of the steel section. The joints take up a proportion of the material (5 to 15%) without adding to the overall width of the sheetpile. Joints can be located on either the flange or web of the sheetpile. Some joint systems reverse the sheetpile section, to create a `double` depth sheetpile profile.
Any disengagement between adjacent sheetpiles breaks up the overall integrity of the sheetpiles, leading to a failure of the sheetpiling. The forces/movements on the joints in conventional sheetpiling can be divided into (a) tensile forces/movements (Ft) occuring from flexure of the sheetpiling, curvature in the sheetpile alignment and/or uneven earth/groundwater forces, (b) Compression forces/movements (Fc) occurring from flexure of the sheetpiling on concave alignments or at corners and (c) outward forces (Ft) from the plane of the sheetpiling, mainly due to uneven earth or groundwater loads and secondary effects from any tensile forces/movements. On the `wider` (w&gt;800) and `deeper` (d&gt;300) profile substantial compressor/tension forces can develope across the joints from the load distribution on and across the pile. These lateral loads have been a limiting factor on the profile of `wider` and `lighter` sheetpiles (w/t&gt;100). The adequacy of the various joint system under these forces varies widely, with only `claw-paw` interlocking joints in conventional sheetpiling covering all force-movement conditions.
Jointing systems adopted on light sheetpiling are loose, with clearances exceeding 5 mm due to fold constraints for the steel sheets. Open joints fill with debris during driving which has to be displaced by the next sheetpile. This obstruction of the joint track causes opening of the joints and leads to disengagement of the sheetpiling. Joint systems adopted on light sheetpiling (+&lt;5 mm) make no provision for the compressor/tension forces developed across the sheetpile, restricting the width (w&lt;500 mm) and depth (d&lt;100 mm) of the sheetpile profiles of light (+=, 5 mm) sheetpiles. Absence of an adequate jointing system has compromised the integrity of light sheetpiling since its introduction in 1933.
Sheetpiles are usually driven with impact or vibrator pile drivers. Driving forces on conventional sheetpiles (t&gt;5 mm) are usually applied through impact blocks and jaw designs developed for normal steel piles. On light sheetpiling, the pile drivers have been limited to the lighter equipment (Qd&lt;100 kN) using capping plates and/or profiled jaws, where Qd is the dynamic pile driving force.
Studies show that driving of the narrow (ws&lt;600 mm) light (t=&lt;5 mm) sheetpiles are limited by compression and buckling effects in the sheetpile. Lateral stability problems develop once the penetration slows (`refusal`) as the driving force rises rapidly and onset of structural fatigue causes a failure around the top of the pile. While slippage in the pile driver's jaws reduces driving forces, the pile reaches premature refusal at shallower depth. Impact hammers do not overcome these problems as driving stresses are even higher.
Driving problems with light sheetpiles imposes a limit on the pile driving forces (Qd) equivalent to a dynamic force of 100 kN, which corresponds to `small` vibratory pile drivers. This dynamic force allows the `narrower` (ws&lt;600 mm) light sheetpiles to be driven to a reasonable (8 m) depth. However, the `wider` (ws=800 mm) sheetpiles can only be driven to shallow depth (&lt;5 m) beyond which extensive site preparation or predrilling is required to reduce the driving resistance.
A number of light sheetpiles have incorporated a secondary corrugation in the flange of the section profile. This corrugation attempts to accommodate the eccentric driving forces occurring in the sheetpile. Depth of the secondary corrugation has been limited to half the section depth (&lt;0.5*d). However, this stiffening of the flange has not solved eccentric load or driving problems except on shallow section profiles (d&lt;80 mm).
Light sheetpiling is very flexible and hence the sheetpiles tend to wander off-line during driving. In moderate to hard driving conditions, the sheetpiles profile distorts and may disengage from the preceding pile due the `weak` joint systems available for light (t=&lt;5 mm) sheetpiling. These effects become appreciable once the effective sheetpile width (ws) exceeds 600 mm, wiht driving tolerance being poorer than +-100 mm on the long (D&lt;7 m), wider (ws&gt;800 mm) sheetpiles. Narrower (ws&lt;600 mm) light sheetpiles can achieve reasonable driving tolerance (+-25 mm) with interlocked joints even on long (+7 m) sheetpiles.
A gradual wander of the sheetpile off-line is difficult to identify as no method exists of checking the final alignment of the sheetpiles before excavation. This alignment problem has hampered the use of light sheetpiles in permanent works and reinforces the limits (w & ws&lt;600 mm) found in the section profiles of light (t=&lt;5 mm) sheetpiling.
The lateral forces on sheetpiling depend mainly on groundwater pressures in the ground behind the sheetpiling. Thus the pressure of groundwater usually compromises the integrity of wide (ws&gt;800) and light (t&lt;5 mm) due to the build up of internal stresses from lateral loads developed on and within the sheetpile profile. These loads create rotational movements and buckling effect that deflects the profile and cause opening of joints in the sheetpile. Normal practice requires installation of lateral drains, deep (&gt;10 m) wells or shallow (8 m) well points. These measures require the sheetpiling to be relatively water-tight so that water drains towards the drains or wells rather than exiting through the joints in the sheetpiling. However, draw down of the ground water may initiate subsidence in the ground behind the sheetpiling. This conflict between preserving ground water levels and the control of ground water pressure severely hampers the use of light (t&lt;5 mm) sheetpiling.
The heavier tracks of conventional (t&gt;5 mm) sheetpiling provides a relatively tight track which can be progressively sealed up with caulking or rubber sealants. However, the open joints occurring in light (t=&lt;5 mm) sheetpiling cannot be effectively sealed. This problem effects the wide light sheetpiles on which alignment tolerances are poor. During driving, the simple `overlap` joint systems separate and even `hooked` or `interlocked` joints may disengage leading to open joints in light (t=&lt;5 mm) sheetpiling. Thus the groundwater has to be drawn down to a level well below the excavation level. This appreciably adds to the site dewatering costs and requires access to the area behind the sheetpiling. In all cases the dewatering of the ground behind the sheetpiling is a separate construction activity. In the past, this drainage of ground water has lead to piping erosion undermining the sheetpiling and caused excessive settlements in the ground behind the sheetpiling.